Sachen 8259

Sachen 8259 describes a series of boards by Sachen. The following information was gathered primarily from FCEU-mm and while it appears to be a good enough behavioral description to run many ROMs, it is likely incomplete.

Variants

There are four known variants of the board. The B variant is described below, as well as the A and C variants which alter behavior by rewiring CHR banks. The D variant behaves much differently and is described in iNES Mapper 137.

Mode and Mirroring Select (Internal 7)

When simple mode is enabled, mirroring is fixed to V, ignoring the m bits, and all 4 CHR banks use the CHR bank 0 register.

CHR Rewiring (A and C variants)

The B variant described above has 6 bit CHR regs covering 2KiB blocks, giving a CHR ROM total of up to 128KiB. The A and C variants rewire the board to increase this limit to 256KiB and 512KiB respectively. This is done by shifting up the CHR ROM address outputs on the mapper and connecting the lower lines directly to the PPU. To be specific:

This can be emulated by shifting the CHR regs left 1 or 2 bits, and inserting the appropriate 1 or 2 bits of PPU address in the low bits. Note that when s = 1, CHR reg 0 is used for all banks, but the extra address line(s) still leak through on the A and C boards. This means that on a C board, when s = 1, the board can be considered to have a single 8KiB CHR ROM bank for pattern tables.